Cyclone furnaces



March 9, 1965 J. H. KIDWELL ETAL 3,172,395

CYCLONE FURNACES Filed March 18, 1963 4 Sheets-Sheet 1 INVENTORS John H. Kidwell Nicholas P. Rusanowsky r E. Lowe March 1965' J. H. KIDWELL ETAL 3,

CYCLONE FURNACES 4 SheetS -Sheet 2 Filed March 18, 1963 March 9, 1965 Filed March 18, 1963 J. H. KIDWELL ETAL. 3,172,395

CYCLONE FURNACES 4 Sheets-Sheet 3 FIG.3

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March 9, 1965 J. H. KIDWELL ETAL 3,172,395

CYCLONE FURNACES Filed March 18, 1963 4 Sheets-Sheet 4 United States Patent 3,172,395 CYCLONE FURNACES John H. Kidwell, Alliance, and Robert E. Lowe and Nicholas P. Rusanowsky, Akron, Ohio, assignors to The Babcock & Wilcox Company, New York, N.Y., a corporation of New Jersey Filed Mar. 18, 1963, Ser. No. 265,715 Claims. (Cl. 122-235) This invention relates in general to vapor generators and more particularly to improvements in the combustion air distribution system for a vapor generator fired by a plurality of cyclone furnaces generally of the type disclosed in US. Patent No. 2,594,312.

In recent years, the steady increase in the capacity of cyclone furnace-fired vapor generating units has resulted in a corresponding increase in the number of cyclone furnaces required per unit. The customary practice in the design of units of the above described character has been to provide separate air conduits or ducts, branching from a main air duct to supply air to the primary, secondary and tertiary air systems of each cyclone furnace. This practice is represented, for example, in US. Patent Nos. 2,902,982 and 2,033,177. While satisfactory operation is obtained with the duct work so arranged, experience has shown that installation of separate air ducts, particularly individual secondary air ducts, to each cyclone furnace is very difficult and costly.

Thus the present invention is directed to improvements in the combustion air supply system for a vapor generator fired by several cyclone furnaces to the end of reducing the cost of the air ductwork, especially the secondary air duct-work, and simplifying the fabrication and erection thereof. In accordance with the invention the secondary air ports of the cyclone furnaces are enclosed by a common windbox arranged to receive the secondary air supply for the cyclone furnaces, with special provisions for effecting an equal distribution of this air supply to the secondary air ports of the cyclone furnaces.

The various features of novelty which characterize our invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, it operating advantages and specific objects attained by its use, reference should be had to the accompanying drawings and descriptive matter in which we have illustrated and described a preferred embodiment of the invention.

Of the drawings:

FIG. 1 is a sectional side elevation of a cyclone furnace fired Vapor generating unit constructed in accordance with the invention;

FIG. 2 is an end view, partly broken away, of the cyclone furnaces;

FIG. 3 is a sectional plan view taken along the line 33 of FIG. 1;

FIG. 4 is an enlarged view of the cyclone furnace secondary air supply apparatus shown in FIG. 1; and

FIG. 5 is a vertical view taken along the line 55 of FIG. 4.

While the combustion air system illustrated and hereinafter described is specifically designed and particularly adapted for a vapor generator fired by a pair of cyclone furnaces, it will be understood that the combustion air system illustrated may be adapted for use in a vapor generator fired by more than two cyclone furnaces.

The steam generating unit illustrated in part in FIGS. 1 and 3 comprises an upright furnace chamber of substantially rectangular horizontal cross-section defined in part by front 11, rear 12 and side walls 13, and a floor 14, with the boundary walls of the furnace chamber being formed in most part by insulation covered 7 ice metallic casing lined by fluid heating tubes secured thereto and being so arranged as to provide a small volume lower furnace portion 10A of substantially uniform horizontal cross-sectional area throughout its height and a large volume upper furnace portion 10C connected to the lower furnace portion by an intermediate furnace portion 10B of continuously increasing horizontal crosssectional area in the direction of the upper furnace portion. Front wall 11 extends vertically at its lower and upper furnace portions and slopes upwardly and outwardly at its intermediate furnace portion. The fuel firing section consists of two independently operable horizontally extending cyclone type furnaces 14 of relatively small volume and boundary wall area disposed at the same level on wall 11 at the lower portion of the furnace chamber 10 and preferably fired by a crushed or granular fuel such as bituminous or semi-bituminous coal. Each cyclone furnace is arranged to burn solid fuel at hight rates of heat release and to separately discharge high temperature gaseous products of combus tion and separated ash residue as a molten slag into the lower portion of the chamber 19. Floor 14 is formed with a centrally disposed opening 16 for the discharge of molten slag to a slag tank, not shown.

The fluid circulation system of the unit comprises a row of upwardly extending parallel tubes 17 lining the front wall 11 of the furnace 10 and having their lower ends connected to a supply header 1S and their upper ends connected to a steam and water drum, not shown; a row of upwardly extending parallel tubes 19 lining the floor 14 and rear wall 12 of the furnace 10 and having their lower ends connected to a supply header 21 and their upper ends connected to the steam and water drum; a row of upwardly extending parallel tubes 24 lining each side wall 13 and having their lower ends connected to a header 26 and their upper ends communicating with the steam and water drum; and a pair of downcomers 22 leading for the steam and water drum and connected for supply of fluid to the headers 18 and 21, and 26 by tubes 23.

In the fuel feeding system for each cyclone furnace, coal from a hopper or bunker, not shown, is supplied to a feeder not shown, arranged to discharge the fuel at a regulated rate to a crusher 28 at the entrance of which the raw coal is joined by a supply of hot high pressure primary air which transports the fuel through the crusher 28 to the primary burner of the corresponding cyclone furnace.

Each cyclone comprises a substantially cylindrical combustion chamber 29 arranged with its major axis horizontal, the circular boundary wall being formed by closely spaced studded tubes 31 covered by a layer of refractory, each tube having a semi-circular bent portion and adjacent tubes having their bent portions oppositely arranged to form the circumferential wall. Tubes 31 along one side of each combustion chamber have their inlet ends connected to a horizontal lower header 32 and their discharge ends connected to a horizontal upper header 33 while tubes 31 along the opposite side have their inlet ends connected to the header 32 and their discharge ends connected to a horizontal upper header 34. The outer end portion of each combustion chamber is provided with an outwardly tapering frusto-conical extension 29A formed by closely spaced studded tubes 31A covered by a layer of refractory material and having their opposite ends connected to horizontally arranged top and bottom headers 36 and 37. Headers 32 and 37 of the cyclone furnaces are connected for supply of fluid from the downcomers 22 by supply tubes 38 while headers 33, 34 and 36 are connected for discharge of fluid to the steam and water drum of the unit by riser tubes 39 extending upwardly through the sloping portion of the wall 11 in sealing relation therewith and through the upper furnace portion C.

Wall tubes 31 along one side of each combustion'chamber 29 are bent radially outwardly along an involute curve for a major portion of the combustion chamber length starting at the inner end of the tapering portion of the chamber, while the corresponding tubes alongthe opposite side are bent outwardly for connection to header 34, thus cooperating, to define an axially elongated secondary air inlet port 41.

The rear end of each combustion chamber is partially closed by the portions of the tubes 17 forming the front wall of the lower furnace 10A, these tube portions being studded and covered with refractory material and having intermediate bent parts forming for each combustion chamber an inwardly projecting throat 42 providing a gas outlet 43 of circular cross-section flaring toward its discharge end and arranged coaxially of the corresponding combustion chamber. Each combustion chamber is provided with a slag outlet formed in thepwall 11 at a position below the corresponding throat 42 for the continuousdischarge of molten slag.

An air and fuel inlet chamber or primary burner 44 of substantially circular cross-section of smaller diameter than the combustion chamber is arranged at the front end of and connected to the frusto-conical extension 29A of each combustion chamber and concentrically opens to the combustion chamber. The primary air-fuel stream discharging from each crusher 28 is directed tangentially into the corresponding primary burner 44 at a high angular velocity for. whirling movement therethrough to the cornbustion chamber by a conduit 46 having an involute curved discharge portion which opens tangentially into the lower side portion of the burner. The effective flow area of the conduit 46, and thereby the velocity of the fuel-air stream, is controlled by a manually operated damper 47.

A circular tertiary air chamber 52 is arranged on the outer end of and concentrically with the burner 44 of each combustion chamber 29. Preheated air is supplied to the chamber 52 by a conduit 53 having an involute curved connection to the chamber 52 producing awhirling stream of tertiaryair which is directed axially of the burner and inwardly of the whirling stream of primary air and fuel entering the corresponding combustion chamber 29. The effective flow area of each conduit 53 is controlled by a damper 48.

In accordance with the invention, air supply provisions for the cylone furnaces comprises insulation covered metallic casing suitably connected to the front wall 11 of the furnace chamber 10 and surrounding and spaced from the combustion chambers of the cylone furnaces to form a chamber 56 including side walls 57, a floor 58 connected to the lower end of the front wall 11, and a front wall 59 formed with a pair of openings encircling and sealingly connected to the small ends of the frusto-conical extensions 29A of the combustion chambers. Thus the primary burners 44 and tertiary air chambers 52 of the cyclone furnaces are disposed outside of the chamber 56.

Chamber 56 is partitioned by horizontal metallic plates 60 which cooperate with front wall 11 and boundary walls of the combustion chambers and chamber 56 to form a windbox 62 enclosing the secondary air ports 41 for common supply of air to the secondary air ports, and a compartment. 63 enclosing the headers 18, 21, 32 and the supply tubes leading thereto. Plates 60 are disposed at the same level as. the horizontal axes of the cylone furnaces and are connected to boundary walls of the combustion chambers 29 and chamber 56 and to the front wall 11. With the chamber '56 so arranged and partitioned, there is no need for insulation of the headers and supply and riser tubes within the chamber 56; the use of air-tight seals at the point where the supply tubes enter the metallic casing-forming chamber 56 is'avoidcd; and

A no metallic casing or refractory is required on the outside of the circumferential walls of the combustion chambers, thereby simplifying repairs to the pressure parts of and a duct 68 to a pair of branch ducts 69 extending along the sides of the chamber 10 and having their discharge ends opening into. opposite sides of the windbox 62 through the side walls 57 thereof.

A part of the preheated air supply flowing through the ducts 69 is discharged through branch conduits 71, controlled by dampers 33, to the inlet ends of the crushers 23 and constitutes the primary air supply; another part of the preheated air supply is discharged through conduits 72 to the inlet ends of the conduits 53 and constitutes the tertiary air supply; while the remaining preheated air supply is discharged into the windbox 62 and constitutes the secondary air supply.

The primary burners 44 and secondary air ports 41 are so arranged relative to the corresponding combustion chambers as to cause the combustion constituents generated in the left cyclone furnace, as viewed when facing the outside of, the front wall 11, to whirl in a counterclockwise direction within the corresponding combustion chamber and the gases generated in the right cyclone furnace to whirl in a clockwise direction. With this arrangement the secondary air ports 41 of the combustion chambers are disposed directly opposite each other'within the windbox 62.

With particular reference to FIGS. 2 and 4, the secondary air port 41 of each of the combustion chambers is occupied by the inner end of a conduit 73 of rectangular cross-section inclined at a slight angle to the horizontal and having its outer'end connected to angles74 forming an extension of the conduit 73 and providing a seat for a shut-off damper 76 and an inlet 77 tothe conduit 73 of substantially the same rectangular cross-sectional area as the conduit 73. Damper 76 is used only when'removing acyclone furnace from service and is not used to regulate the quantity of air for combustion. The inner end of conduit 73 is provided with a plate damper '75 for controlling the velocity of the corresponding secondary air streamwhile maintaining the entering air at all times along the combustion chamber wall. Each plate damper is mounted on a shaft 89 which is manually operated by an arm, not shown, disposed outside the windbox 62. Damper 76 is enclosed by a housing 78 suitably connected to the angles 74 and within which is disposed a rectangular conduit 79 of substantially the same cross-sectional area as the conduit 73 formed by side and bottom surfaces of the housing 78 and by a plate 81 disposed in and connected to the housing. Conduit 79 extends coaxially of the inlet 77 and'conduit 73, has arc-shaped top'and bottom sides at its air intake end to provide a bell-shaped inlet 82 flaring in the direction of the incoming secondary air, and has its discharge end spaced from the inlet 77 to permit vertical movement of the damper 76. Vertical movement of the damper 76 is effected by a rack-and-pinion system enclosed by the housing 78 and comprising a horizontal shaft 84 driven by a motor (not shown) outside the windbox'and carrying a pair of pinions' 87 arranged to mesh with corresponding racks 38 connected to opposite ends of the damper 76.

The inlet end of conduit 79 of each combustion chamber is connected to a horizontal air distribution hood 89' extending along the entire width of the conduit 79, having a semi-cylindrically shaped portion 89A formed at its edges with tangential extensions 89B connected to the flaring inlet end of the conduit 79, and formed with uniformly spaced circular holes 91 throughout its extent for passage of secondary air from the windbox 62 to the corresponding combustion chamber byway of the conduits 73 and 79. The perforated hoods 89 and flared inlets of the conduits 79 are proportioned and arranged to provide a pressure drop between the windbox and throats of the flared inlets of a magnitude sufiicient to assure a substantially equal distribution of the supply of air entering the windbox to the secondary air ports 41 of the cyclone furnaces. With the secondary air supply system for the cyclone furnaces so constructed and arranged, test work has demonstrated that the pressure difference between the windbox and the throat of the flared inlet 82 of each combustion chamber will be directly proportional to the rate of fiow of secondary air passing to the corresponding secondary air port regardless of the approach conditions of the air to the hood of the corresponding combustion chamber. While these tests have indicated that even distribution of secondary air is attained with the above-described system, provisions are made for measuring the difference between pressures at the windbox 62 and at the throat of the flared inlet of the conduit 79 of each combustion chamber to determine the quantity of air flowing to each combustion chamber so that in the event of unbalanced distribution adjustment can be made by slight alteration of the position of either or both dampers 75 of the respective combustion chambers. This measurement may be made by providing openings in the throats of the flared inlets of the conduits 79 and by a perforated pipe extending within and across the width of the windbox 62. The pressures measured at these points are transferred to suitable metering devices, not shown.

By way of example and not of limitation, in the secondary air distribution system for 9 foot diameter cyclone furnaces, conduits 73 and 79 of each cyclone furnace have an inside width of 7 feet 6 inches and an inside height of 12 /2 inches, hood portion 89B has a length of 4 /2 inches hood portion 89A has a radius of 9% inches and each of the arc-shaped top and bottom sides of inlet 82 has a radius of 3 inches. The perforated plate forming the hood has 57% open area with .469 inch diameter holes on inch centers.

In operation of the cyclone furnaces and the air and fuel supply construction described, preheated air is supplied at a high positive pressure to the duct 68 wherein it divides about equally for flow through the branch ducts 69. The flow of air through each duct 69 is split into three streams by suitable regulation of the dampers 48, 83, 75, 15-20% being used as primary air, 7580% as secondary air and 3-5 as tertiary air, with the primary and tertiary air stream passing to the corresponding cyclone furnace and with the secondary air stream passing to the windbox 62. The total air supplied preferably ranges from about 1051l5% of the theoretical combustion requirements. The primary air-fuel stream passing from each crusher 28 tangentially enters the chamber 44 of the corresponding combustion chamber and whirls therethrough in a high velocity stream to the combustion chamber, while the corresponding tertiary air stream whirls through the chamber 44 in the same direction of rotation as and generally and axially of the whirling stream of primary air and fuel.

The streams of high velocity secondary air discharging from the conduits 69 to the windbox 62 combine in the windbox and constitute a common supply for the second ary air ports 41 of the combustion chambers, with the air distribution hoods 89 of the combustion chambers imposing a resistance to air flow which has a proportioning and equalizing effect on the amount of air passing from the windbox to the secondary air ports of the combustion chambers. The streams of high velocity secondary air discharged from the conduits 73 tangentially enter the corresponding combustion chambers in the same direction of rotation as and at the outer side of the whirling burning streams of primary air and fuel. With the air streams and fuel entering each combustion chamber as described, combustion of the fuel and air progresses at a rapid rate,

which increases in the zone of secondary air admission, with a gradual mixing of the secondary air stream with the enclosed streams of primary air and fuel. Combustion proceeds at a rate sufficient to maintain a normal mean temperature in each combustion chamber substantially above the fuel ash fusion temperature over a wide range of operation. Under such combustion conditions the ash content of the fuel is rapidly reduced to a molten condition and due to the centrifugal effect thereon, the combustion chamber walls will become coated with molten slag which adheres to the refractory inner face of the Walls and provides a sticky surface against which fuel particles are thrown and to which they adhere. The high velocity of the burning fuel-air mixture causes the gas stream to follow a helical path to the rear of the combustion chamber where the gas is caused to reverse direction before entering the gas outlet 43. Molten slag resulting from combustion continuously discharges through the outlet 40 of each combustion chamber into the furnace chamber 10 for flow through the outlet 16 to a slag tank. The gases discharged through each outlet 43 into the furnace chamber 10 contain little, if any, combustible, combustion of the fuel being substantially completed in the corresponding combustion chamber.

When both cyclone furnaces are in operation the heat input to each is the same. Through volumetric metering of the fuel the same quantity of fuel is fed to each cyclone furnace. Thus it follows that to burn this fuel each cyclone furnace requires the same quantity of com bustion air. In order to maintain the most efficient recovery of ash as slag in each cyclone furnace and also to make sure the slag flows freely from each cyclone furnace the firing rate should not be less than one-half load per cyclone furnace for any extended period of time. It the vapor generating rate is such that the cyclone furnaces will be operating at less than one-half full load, one of the cyclone furnaces should be removed from service so that the load on the remaining cyclone furnace is at least one-half full load.

While the instant invention has been disclosed with reference to a particular embodiment thereof, it is to be appreciated that the invention is not to be taken as limited to or of the details thereof as modifications and variations thereof may be made without departing from the spirit or scope of the invention.

What is claimed is:

1. In a vapor generator, walls forming a furnace chamber, walls forming a plurality of separate combustion chambers of substantially circular cross-section each connected to the furnace chamber and formed with a gas outlet opening to the furnace chamber, means for introducing fuel into each combustion chamber and affecting a whirling path of travel therein along the circumferential wall thereof, and means for supplying combustion air to said combustion chambers, said last named means including an axially elongated air inlet arranged tangentially to the circumferential wall of each combustion chamber, means forming a windbox connected to said furnace chamber and enclosing the air inlet of each of said combustion chambers and providing a common source of supply of combustion air for said combustion chambers, means for supplying combustion air to said windbox, and means providing substantially equal distribution of the air flowing through said windbox to the air inlets of said combustion chambers, said last named means including a perforated air distribution hood connected to the air inlet of each combustion chamber, with the hoods being proportioned and arranged to impose a resistance to air flow sufficient to provide substantially equal distribution of the air passing from the windbox to the air inlets of the combustion chambers.

2. In a vapor generator, walls forming a furnace chamber, a pair of cyclone furnaces each having a combustion chamber of substantially circular cross-section connected to the furnace chamber and formed with a restricted gas outlet opening to the furnace chamber, means for introducing fuel into each combustion chamber and effecting a whirling path of travel threin along the circumferential wall thereof, and means for supplying combustion air to said combustion chambers, said last named means in cluding an axially elongated air inlet arranged tangentially to the circumferential wall of each combustion chamber, means forming a windbox connected to said furnace chamber and enclosing the air inlet of each of said combustion chambers and providing a common source of supply of combustion air for said combustion chambers, means for supplying combustion air tosaid windbox, and means providing substantially equal distribution of the air flowing through said windbox to the air inlets of said combustion chambers, said last named means including a perforated air distribution hood connected to the air inlet of each combustion chamber, with the hoods being proportioned and arranged to impose a resistance to air flow suflicient to provide substantially equal distribution of the air passing from the windbox to the air inlets of the combustion chambers.

3. In a vapor generator, walls forming a furnace chamber, a pair of cyclone furnaces each having a combustion chamber of substantially circular cross-section connected to the furnace chamber and formed with a restricted gas' outlet opening to the furnace chamber, means for introducing fuel into each combustion chamber and effecting a whirling path of travel therein along the circumferential wall thereof, and means for supplying combustion air to said combustion chambers, said last named means includ- 5 ing an axiallyelongated air inlet arranged tangentially to the circumferential wall of each combustion chamber, means forming a windbox connected to said furnace chamber and enclosing the air inlet of each of said combustion chambers and providing a common source of supply of combustion air for said combustion chambers, means for supplying combustion air to said windbox, and means providing substantially equal distribution of the air fiowing through said windboX to the air inlets of said combustion chambers, said last named means including a conduit disposed in the air inlet of each ofsaid combustion chambers and having one end opening to the corresponding air inlet and its opposite end flared in the direction of the incoming combustionair, and an airflow distribution hood connected to the flared end of each conduitand formed with perforations opening to said windbox.

4. In a vapor generator, walls forming a furnace chamber, a plurality of cyclone furnaces each having a combustion chamber of substantially circular cross-section connected to the furnace chamber and formed with a restricted gasoutlet opening to the furnace chamber, means for introducing fuel into each combustion chamber and effecting a whirling path of travel therein along the circumferential wall thereof, and means for supplying combustion air to said combustion chambers, said last named means including an axially elongated air inlet arranged tangentially to the circumferential wall of each combustion chamber, means forming a windbox connected to said furnace chamber and enclosing theair inlet of each of said combustion chambers and providing a common source of supply of combustion air, for said combustion chambers, means for supplying combustion air to said windbox, and; means providing substantially equal dis tribution of the air flowing through said windbox to the air inlets of said combustion chambers, said last named means including a conduit disposed in the air inlet of each of said combustion chambers and having one end opening to the corresponding air inlet and its opposite end flared in the direction of the incoming combustion air, and a semi-cylindrical air flow distribution hood connected to the flared end of each conduit and formed with perforations opening to said windbox.

5. In a vapor generator having a natural circulation fluid circulation system, walls including vapor generating tubes forming a furnace chamber, walls including vapor generating tubes forming a plurality of separate combustion chambers of substantially circular cross-section each connected to said furnace chamber and having a restricted gas outlet opening thereto, means for introducing fuel into each combustion chamber and effecting a helical path of travel therein along the circumferential Wall thereof, means connecting said vapor generating tubes into said fiuid circulation system including supply tubes connected for flow of fluid to the inlet ends of the tubes of the combustion chambers and riser tubes connected for flow of fluid from theoutlet ends of the tubes of the combustion chambers, and means for supplying combustion air to said combustion chambers, said last named means including an axially elongated air inlet arranged tangentially to the circumferential wall of each combustion chamber, means forming a chamber connected to said furnace chamber and enclosing said combustion chambers, partition means dividing said chamber into a compartment enclosing said supply tubes and a windbox enclosing said riser tubesv and the air inlets of said combustion chambers and providing a common source of supply of combustion air to said windbox, and means providing substantially equal distribution of the air flowing through said windboX to the air inlets of said combustion chambers, said last named means including i a perforated air distribution hood connected to the air inlet of each combustion chamber, with the hoods being proportioned and arranged to impose a resistance to air flow sufficient to provide substantially equalrdistribution of the air passing from the windbox to the air inlets of the com bustion chambers.

References Cited in the file of this patent UNITED STATES PATENTS 2,800,114 Kolling July 23, 1957 3,043,279 Blomquist July 10, 1962 3,052,221 Rosahl Sept. 4, 1962 

1. IN A VAPOR GENERATOR, WALLS FORMING A FURNACE CHAMBER, WALLS FORMING A PLURALITY OF SEPARATE COMBUSTION CHAMBERS OF SUBSTANTIALLY CIRCULAR CROSS-SECTION EACH CONNECTED TO THE FURNACE CHAMBER AND FORMED WITH A GAS OUTLET OPENING TO THE FURNACE CHAMBER, MEANS FOR INTRODUCING FUEL INTO EACH COMBUSTION CHAMBER AND AFFECTING A WHIRLING PATH OF TRAVEL THEREIN ALONG THE CIRCUMFERENTIAL WALL THEREOF, AND MEANS FOR SUPPLYING COMBUSTION AIR TO SAID COMBUSTION CHAMBERS, SAID LAST NAMED MEANS INCLUDING AN AXIALLY ELONGATED AIR INLET ARRANGED TANGENTIALLY TO THE CIRCUMFERENTIAL WALL OF EACH COMBUSTION CHAMBER, MEANS FORMING A WINDBOX CONNECTED TO SAID FURNACE CHAMBER AND ENCLOSING THE AIR INLET OF EACH OF SAID COMBUSTION CHAMBERS AND PROVIDING A COMMON SOURCE OF SUPPLY OF COMBUSTION AIR FOR SAID COMBUSTION CHAMBERS, MEANS FOR SUPPLYING COMBUSTION AIR TO SAID WINDBOX, AND MEANS PROVIDING SUBSTANTIALLY EQUAL DISTRIBUTION OF THE AIR FLOWING THROUGH SAID WINDBOX TO THE AIR INLETS OF SAID COMBUSTION CHAMBERS, SAID LAST NAMED MEANS INCLUDING A PERFORATED AIR DISTRIBUTION HOOD CONNECTED TO THE AIR INLET OF EACH COMBUSTION CHAMBER, WITH THE HOODS BEING PROPORTIONED AND ARRANGED TO IMPOSE A RESISTANCE TO AIR FLOW SUFFICIENT TO PROVIDE SUBSTANTIALLY EQUAL DISTRIBUTION OF THE AIR PASSING FROM THE WINDBOX TO THE AIR INLETS OF THE COMBUSTION CHAMBERS. 